JP3362900B2 - Surface emitting device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface emitting device, and more particularly to a surface emitting device which can be used as a backlight of a liquid crystal display device.

As a surface emitting device used as a backlight of a liquid crystal display device, it is required that the entire surface has uniform brightness, is thin, and is light.

[0003]

2. Description of the Related Art FIG. 32 shows a surface emitting device 10 disclosed in Japanese Patent Laid-Open No. 2-157791. The device 10 is composed of a plate 11 and fluorescent tubes 12 on both sides of the plate 11.

The plate 11 is a light guide plate 13 made of acrylic resin.
The main body of which is a light diffusing plate 14 and a diffuse reflection pattern 1
5 and the reflector 16 are added.

The light guide plate 13 has a function of confining light, and the diffuse reflection pattern 15 has a function of emitting light from the light guide plate 13.

[0006]

Since the light guide plate 13 which is the main body of the plate body 11 is a plate made of acrylic resin, the surface emitting device 10 is considerably heavy.

Therefore, an object of the present invention is to provide a surface emitting device which utilizes a prism plate and has a hollow structure for confining light in the space below the prism plate, which realizes a significant weight reduction.

[0008]

FIG. 1 shows the principle configuration of a surface emitting device 20 of the present invention.

The surface emitting device 20 includes a light source 21 and a light source 21.
A prism plate 24 whose upper surface is a light-emitting surface 24a by reflecting the light 22 from the inside and confining the light in the lower space 23,
It comprises a reflection control means 25 for controlling so as to reduce reflection by the prism plate 24.

[0010]

The prism plate 24 acts so as to confine the light 22 in the space 23.

The reflection control means 25 operates so that the light is emitted from the prism plate 24 as indicated by reference numeral 23a and the luminance distribution of the light emitting surface 24a is controlled.

[0012]

The present invention is roughly classified into a first invention and a second invention.

According to a first aspect of the invention, the reflection control means is provided on the prism plate itself. A second aspect of the invention has a configuration in which the reflection control means is provided separately from the prism plate.
First, an embodiment of the first invention will be described.

[First Embodiment of the First Invention] FIG. 2 shows a surface emitting device 30 according to a first embodiment of the first invention.

Reference numeral 31 is a main body, and U-shaped holder portions 31a and 31b are formed on both sides.

On the bottom surface of the main body 31, there is arranged a reflection plate 32 formed by sticking a silver thin film on the surface of an aluminum sheet having a thickness of 0.5 mm.

Reference numerals 33 _-1 and 33 _-2 are cold cathode fluorescent lamps having an outer shape of 3φ × 240 mm and the holder portions 31a and 31b.
They are provided inside and face each other.

Reference numeral 34 denotes a prism plate, which is a holder portion 31.
It is provided flat across a and 31b, and the main body 31
Covers the upper opening of the.

The prism plate 34 is a plate made of transparent polycarbonate resin having a thickness of 0.36 mm, and is shown in FIGS.
As also shown in FIG.
The upper surface is a flat surface 34b.

The prism surface 34a has a shape in which linear prism portions 34c having an apex angle θ of 90 degrees are arranged at a pitch p = 0.35 mm. The prism angle (base angle) α of the linear prism portion 34c is 45 degrees.

Reference numeral 34d denotes a minute cylindrical portion, which is the prism surface 3
It is formed by being distributed in a dot shape on 4a.

Each minute cylindrical portion 34d has a linear prism portion 3
The shape of 4c is disturbed, the light confining function described later by the linear prism portion 34c is lowered, and the reflection control means is configured.

The minute column portion 34d is formed by printing an ultraviolet-curable acrylate-based transparent resin in a dot pattern on the prism surface 34a and then curing the resin.

As shown in FIG. 2, the prism plate 34 has the prism face 34a on the lower side, and the longitudinal axis 35 of the linear prism portion 34c when viewed from above has the cold cathode tube 3 formed therein.
It is mounted in a direction orthogonal to the longitudinal axis 36 of 3 _-1 (33 _-2 ).

The space 37 is below the prism plate 34. Therefore, the device 30 is about 1/3 of the weight of the conventional device using the light guide plate (however, the value varies depending on the configuration) and is light.

Reference numeral 38 is a light diffusion plate having a thickness of 0.25 m.
The glass beads are scattered on the transparent resin film of m, and are placed on the prism plate 34.

Reference numeral 40 is a light emitting surface.

Next, the operation of the surface emitting device 30 having the above structure will be described.

Operation of the linear prism 34c of the prism plate.

The light 39 emitted from the cold cathode tubes 33 _-1 , 33 _-2 is reflected by the slopes of the adjacent linear prism portions 34c as shown by reference numerals 39a, 39b in FIG. Returned to.

As a result, the light rays directed to the portion of the prism plate 34 where the shape of the linear prism 34c is not disturbed are confined in the space 37.

FIG. 6 shows the result of an experiment conducted by the present inventor.

In the experiment, the emission rate from the prism plate was obtained by measuring the total luminous flux of a surface emitting device in which a general prism plate was mounted by appropriately changing its direction.

Line I is the emission rate when the longitudinal axis of the linear prism portion is parallel to the axis of the cold cathode tube, line II.
Shows the emission rate when the linear prism portion is oriented such that its longitudinal axis is orthogonal to the axis of the cold cathode tube.

The emission rate of 100% is a value when the prism plate is removed, and means that the confined light is zero. The emission rate of 0% is a value when an opaque plate is placed instead of the prism plate, and means that all the light is confined. For example, an emission rate of 80% means that 20% of light is confined.
From the lines I and II, when the longitudinal axis of the linear prism portion is oriented orthogonal to the axis of the cold cathode tube and the prism angle α is 45 degrees, the emission rate is as shown by the point P _1. It is the lowest at about 65%, which shows that the light is most effectively confined. The direction of the prism plate and the prism angle of the device 30 are determined based on the above experimental results, and approximately 35% of the light emitted from the cold cathode tubes 33 _-1 , 33 _-2 is stored in the space 37. Light is trapped.

A part of the light (65% of the light) is shown in FIG.
Inside, as shown by the reference numeral 39c, it enters the prism plate 34, is diffused by the light diffusion plate 38 as shown by the reference numeral 39d, and is emitted from the surface emitting device 30.

Operation of the minute column portion 34d of the prism plate.

In FIG. 5, the light 3 directed to the minute cylindrical portion 34d
Most of 9 enters into the minute cylindrical portion 34d, as indicated by reference numeral 39e.

The entering light 39e is diffused by the light diffusing plate 38 as shown by the reference numeral 39f and emitted from the surface emitting device 30.

As described above, the minute column portion 34d acts so as to reduce the light confining effect of the prism plate and thus the reflection characteristic (transmission characteristic) of the prism plate 34.

When there is no minute cylindrical portion 34d at all, the luminance distribution of the light emitting surface of the surface emitting device is such that the side closer to the cold cathode tubes 33 _-1 , 33 _-2 has the luminance as shown by the line III in FIG. Becomes a high distribution.

In the present embodiment, the minute cylindrical portions 34d are provided in such a distribution that the portion farther from the cold cathode tube has a higher density.

Therefore, the emission rate at the central portion of the light emitting surface is increased, and the luminance distribution of the light emitting surface 40 is substantially uniform over the entire surface, as indicated by the line IV.

Next, a modification of the reflection control means, that is, a prism plate which can be used in place of the prism plate 34 will be described.

The prism plate 34A shown in FIG. 8 has a structure having a minute hole 50 in place of the minute cylinder 34d. The collection of the minute holes 50 constitutes reflection control means,
The reflection characteristic of the prism plate 34A is determined.

The prism plate 34B shown in FIG.
It is a configuration having 1. The collection of the minute cutouts 51 constitutes reflection control means and determines the reflection characteristics of the prism plate 34B.

The prism plate 34C shown in FIG.
It is a structure having. The plane portion 52 constitutes reflection control means and determines the reflection characteristic of the prism plate 34C.

It can be seen from the line II in FIG. 6 that when the prism angle (base angle) is smaller than 45 degrees, the emission rate increases.

The prism plate 34D in FIG. 10 and the prism plate 34E in FIG. 11 are based on the above facts.

The prism plate 34D shown in FIG. 11 has a linear prism portion 34Dc whose prism angle α is gradually reduced as the prism angle α is farther from the end face 34e as shown by the line V in FIG. 12 (α ₁ < α). The linear prism portion 34Dc constitutes reflection control means and determines the reflection characteristic of the prism plate 34D.

The prism plate 34E shown in FIG. 13 has an end face 34e.
For a portion distant from, the prism angle alpha ₂ is an end face 3
The linear prism portion 34Ec having a prism angle α ₂ smaller than the prism angle α of the linear prism portion 34c closer to 4e.
It is a structure having.

The linear prism portion 34Ec constitutes reflection control means and determines the reflection characteristic of the prism plate 34E.

From the line II in FIG. 6 and the line I, it can be seen that the emission rate increases when the linear prism portion 34c is oriented parallel to the cold cathode tube.

The prism plate 34F of FIG. 14 is based on this fact, and has a structure in which a part of the prism plate 34F has a linear prism portion 34Fc oriented parallel to the end face 34e.

The linear prism portion 34Fc is parallel to the cold cathode tube and constitutes reflection control means, and the prism plate 3 is provided.
Determine the reflection characteristics of 4F.

The prism plate 34G shown in FIG. 15 has a structure in which two prism plates 34 shown in FIG. 3 are laminated by being bonded by a transparent resin portion 55 distributed in a dot pattern by printing. If this prism plate 34G is used, the prism plate 3
Compared to the case where there is only one sheet 4, the output rate is reduced and the light confining action is large, and since the minute cylindrical portions 34d made of the transparent resin portion 55 are present in each prism plate 34, the control range of the transmission characteristics is large. .

[Second Embodiment of the First Invention] A surface emitting device 30A shown in FIG. 16 is the same as the structure shown in FIG. 2 except that the prism plate 30H is concavely curved. The same reference numerals are given and the description thereof is omitted.

Distance between prism plate 30H and reflector 32
Separation (D₁, D₂, D₃) Is a cold cathode tube 33_-1(33_-2)
It gradually becomes smaller as it gets away from. That is, D₃
> D ₂> D₁The space 37A goes to the center
It has a narrow wedge shape.

Since D ₃ is small, cold cathode tubes 33 _-1 , 33
_The number of reflections between _-2 and repeated round trips is smaller than in the case of the device of FIG.

When the light is repeatedly reflected, it is absorbed each time it is reflected, and the intensity of the light decreases. For example, the reflectance is 9
Even if it is as high as 4%, if the reflection is repeated 11 times, the light intensity becomes 0.94 ¹¹ ≈0.51, which is about half.

In the device 30A of FIG. 16, since the number of times of reflection of light is small, the decrease in light intensity is suppressed, and the brightness of the light emitting surface 40 is higher than that of the device 30 of FIG.

[Third Embodiment of the First Invention] A surface emitting device 30B shown in FIG. 17 includes two prism plates 30I and 30J.
It is a configuration in which it is provided obliquely as it is in a planar shape.

This device 30B has the same characteristics as the device 30A of FIG. 16, and since the prism plate is not curved, it is easy to manufacture.

[Fourth Embodiment of the First Invention] The surface emitting device 30C of FIG. 18 is different from the surface emitting device 30A of FIG. 16 in that it is made of a transparent polycarbonate resin plate having a thickness of 0.36 mm and has an apex angle of 90. Condensing prism plate 60 having linear prism of degree
Is a structure added with.

The condenser prism plate 60 is the prism plate 34H.
And the light diffusing plate 38 are positioned between the light diffusing plate 38 and the light diffusing plate 38, and the light emitted from the light collecting prism 60 is condensed in a direction perpendicular to the light diffusing plate 38.

As a result, the light emitting surface 40 has higher brightness than the device of FIG.

[Fifth Embodiment of the First Invention] In the surface emitting device 30D of FIG. 19, a condenser prism plate 61 is attached to the device 30B of FIG. 17 (however, the prism plate 34G of FIG. 15 is attached). This is an added configuration. The linear prism portion 62 of the condenser prism plate 61 has the cold cathode tube 33 _-1 when viewed from above.
It has a structure inclined by about 45 degrees with respect to the axis 36.

The thickness is 8 mm, and the light emitting area is 165 × 225.
The luminance distribution of the light emitting surface 40 in mm is as shown in FIG.
As indicated by the line VI, the light emitting surface 40 is substantially uniform and has high brightness.

[Sixth Embodiment of First Invention] The surface emitting device 30E of FIG. 21 has a structure in which a cold cathode tube 33 _-1 is provided on only one side.

Reference numeral 65 denotes a reflector, which is the cold cathode tube 33._-1And
The cold cathode tubes 33 are arranged facing each other. _-1When lights up
In addition, it is considered that reflected light functions as a pseudo different light source.
Can be obtained.

The prism plate 34K is curved in a concave shape. This device 30E is advantageous for high brightness.

[Seventh Embodiment of the First Invention] A surface emitting device 30F shown in FIG. 22 has a main body 31B whose bottom plate 31Ba is inclined.

The prism plate 34 is horizontal.

This device 30F is advantageous for thinning.

[Eighth Embodiment of the First Invention] The surface emitting device 30G shown in FIG. 23 is different from the device 20E shown in FIG. 21 in that prism plates 34L and 34M are tilted from both sides instead of the prism plate 34K. It is the configuration provided.

Incidentally, in the second to eighth embodiments,
The prism plates 34A to 34G shown in FIGS. 8 to 15 may be used instead of the respective prism plates.

Next, an embodiment of the second invention will be described.

[First Embodiment of the Second Invention] FIG. 24 shows a surface emitting device 100 according to the first embodiment of the second invention.

Reference numeral 101 denotes a directional light source, which gives a directivity to a backlight 102 which is a surface light source and a light beam 103 emitted from the backlight 102, and has a directional light beam 10.
The optical fiber array 104 for emitting 3a.

Reference numeral 105 denotes a prism plate, and as shown in FIG. 25, a large number of linear prism portions 106 are closely arranged on the upper surface.

The vertical angle θ ₁ of the linear prism portion 106 is 90 degrees.

The upper surface of the prism plate 105, that is, the prism surface is the light emitting surface 107.

The non-scattered light 109 incident at an angle perpendicular to the lower surface 108 is totally reflected by the inclined surface of the linear prism portion 106 as indicated by reference numerals 109a and 109b, and the light source 1
Returned to the 01 direction.

The scattered light 109c escapes from the linear prism portion 106 as indicated by reference numeral 109d, and the light emitting surface 1
It is emitted from 07.

That is, the prism plate 105 uses the non-scattered light 10
9 functions to be confined in the space 110 below the prism plate 105.

Reference numeral 112 denotes a polymer dispersion type liquid crystal display element, which is provided at a position between the light source 101 and the prism plate 108 and constitutes reflection control means.

The element 112 has a structure in which a liquid crystal material is encapsulated in a polymer microcapsule. When a voltage is applied, the liquid crystal molecules are aligned and the light 103a is denoted by reference numeral 113.
As shown by a, it is transmitted without being scattered.

When no voltage is applied, the liquid crystal molecules are in a disordered state, and the light 103a is scattered as indicated by reference numeral 113b.

Therefore, the element 112 functions so as to break the light confining function of the prism plate 105 when no voltage is applied.

In the device 100 having the above-described structure, the light emitting surface 107 is in a state in which the portion of the element 112 corresponding to the portion to which no voltage is applied is bright and the other portions are in a dark state, and the light emitting surface 107 has a good contrast. Emits light.

That is, the light emitting surface 107 emits light according to the driving state of the element 112 and can be used as a display device.

Next, a modification of the prism plate 105 will be described.

The prism plate described below can be provided in place of the prism plate 105.

The prism plate 105A shown in FIG.
Is a configuration in which the pyramidal prism portions 115 shown in FIG.

The prism plate 105B shown in FIG. 27 has a structure in which a diffusion sheet 116 is stacked on the prism plate 105.

The diffusion sheet 116 is, for example, a GE company lexan film (model number 8B36).

The prism plate 105C shown in FIG. 28 has a low refractive index resin layer 117 on the upper surface of the prism plate 105,
This is a configuration having a diffusion plate 118 integrally.

The diffusion plate 118 is formed by forming the resin layer 117 and then forming an acrylic light diffusion resin integrally with the prism plate 105.

A wide viewing angle characteristic can be obtained by using the prism plates 105B and 105C.

The prism plate 105D shown in FIG.
The prism plate 105B of No. 7 has a light shielding wall 119 added thereto.

The light shielding wall 119 is arranged corresponding to each dot of the element 112, and prevents light emission from being mixed between each dot.

[Second Embodiment of the Second Invention] The surface emitting device 100A of FIG. 30 has a structure in which the prism plate 105 and the element 112 are integrated. This device 100A can be manufactured with higher productivity than the device 100 of FIG. [Third Embodiment of Second Invention] Surface emitting device 100 of FIG.
B has a directional light source 101A including a halogen lamp 120 and a reflector 121.

The prism plate 105E has a prism portion 122.
_{-1 to} 122 _-4 .

The prism plate is the center line 123 of the light source 101A.
It has a shape that is gradually inclined outward as it moves away from.

This is the light ray 103b from the light source 101A.
However, it corresponds to the fact that it is spread a little.

[0106]

As described above, according to the first to fifth aspects of the present invention, since the light is confined in the space below the prism plate, the light guide plate has a structure of about 1% compared with the device using the light guide plate. /
Weight reduction to 3 is possible. The structure having a portion in which the shape of the prism portion is different from the other portions has a structure for controlling the degree of confining light in the space below the prism plate. Therefore, since the structure for controlling the degree of confining the light in the space below the prism plate is incorporated in the prism plate itself, the structure can be simplified.

According to the invention of claim 6 , the light source is a cold cathode tube arranged laterally, and the prism plate has a configuration in which aligned linear prism portions are arranged in a direction orthogonal to the cold cathode tube. Therefore, the reflection effect of the prism plate can be maximized, and the light can be efficiently confined in the space.

According to the invention of claim 7 , the prism plate comprises:
Since the space below the prism plate is provided so as to become narrower as the distance from the cold cathode tube increases, the number of times light is reflected can be reduced, and the intensity of light due to repeated reflection can be reduced. The decrease can be suppressed, and the brightness of the light emitting surface can be correspondingly increased.

[0109]

[Brief description of drawings]

FIG. 1 is a principle view of a surface emitting device of the present invention.

FIG. 2 is a diagram showing a first embodiment of the surface emitting device of the first invention according to the present invention.

FIG. 3 is a perspective view of the prism plate seen from below in FIG.

FIG. 4 is a sectional view taken along line IV-IV in FIG.

FIG. 5 is a diagram illustrating an operation of a prism plate.

FIG. 6 is a diagram showing the relationship between the exit direction and the prism plate orientation and prism angle.

FIG. 7 is a diagram showing a luminance distribution on a light emitting surface of the device of FIG.

FIG. 8 is a diagram showing a first modification of the prism plate.

FIG. 9 is a diagram showing a second modification of the prism plate.

FIG. 10 is a diagram showing a third modification of the prism plate.

FIG. 11 is a diagram showing a fourth modification of the prism plate.

12 is a diagram illustrating a prism angle of a linear prism portion in FIG.

FIG. 13 is a diagram showing a fifth modification of the prism plate.

FIG. 14 is a diagram showing a sixth modification of the prism plate.

FIG. 15 is a diagram showing a seventh modification of the prism plate.

FIG. 16 is a view showing a second embodiment of the surface emitting device of the first embodiment according to the present invention.

FIG. 17 is a third surface-emitting device of the first invention according to the present invention.
It is a figure which shows an Example.

FIG. 18 is a fourth surface-emitting device of the first invention according to the present invention.
It is a figure which shows an Example.

FIG. 19 is a fifth surface-emitting device of the first invention according to the present invention.
It is a figure which shows an Example.

20 is a diagram showing a luminance distribution on a light emitting surface of the device of FIG.

FIG. 21 is a sixth surface-emitting device according to the first aspect of the present invention.
It is a figure which shows an Example.

FIG. 22 is a seventh surface-emitting device of the first invention according to the present invention.
It is a figure which shows an Example.

FIG. 23 is an eighth surface emitting device of the first invention according to the present invention.
It is a figure which shows an Example.

FIG. 24 is a first surface emitting device of a second invention according to the invention.
It is a figure which shows an Example.

FIG. 25 is a perspective view showing a prism plate in FIG. 24.

FIG. 26 is a diagram showing a first modification of the prism plate.

FIG. 27 is a diagram showing a second modification of the prism plate.

FIG. 28 is a diagram showing a third modification of the prism plate.

FIG. 29 is a diagram showing a fourth modification of the prism plate.

FIG. 30 is a second surface-emitting device of the second invention according to the present invention.
It is a figure which shows an Example.

FIG. 31 is a third surface-emitting device of the second invention according to the present invention.
It is a figure which shows an Example.

FIG. 32 is a diagram showing an example of a conventional surface emitting device.

[Explanation of symbols]

10 Surface Emitting Device 11 Plate 12 Fluorescent Tube 13 Light Guide Plate 14 Light Diffusing Plate 15 Diffuse Reflecting Pattern 16 Reflecting Plate 20 Surface Emitting Device 21 Light Source 22 Confined Light 22a Emitted Light 23 Space 24 Prism Plate 24a Light Emitting Surface 25 Reflection Control Means 30, 30A to 30G Surface emitting device 31 Main body 31a, 31b Holder portion 32 Reflector plate 33 _-1 , 33 _-2 Cold cathode tube 34, 34A to 34M Prism plate 34a Prism surface 34b Flat surface 34c, 34Dc, 34Ec, 34Fc Line -Shaped prism portion 34d Micro cylindrical portion 34e End surface 35 Longitudinal axis line 36 of linear prism portion Longitudinal axis line 37, 37A of cold cathode tube Space 38 Light diffusing plate 39 Light ray 40 Light emitting surface 50 Micro hole portion 51 Micro cutout portion 52 Flat surface portion 60, 61 Condensing prism plate 65 Reflecting plates 100, 100A, 100B Surface emitting devices 101, 10 A directional light source 102 backlights 103, 108, 113 light 104 optical fiber arrays 105, 105A to 105E prism plate 106 linear prism portion 107 light emitting surface 110 space 112 polymer dispersed liquid crystal display element 115 pyramidal prism portion 116 diffusion sheet 117 Low Refractive Index Resin Layer 118 Diffusion Plate 119 Light _- Shielding Wall 120 Halogen Lamp 121 Reflectors 122 _{-1 to} 122 _-4 Prism 123 Center Line

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Daiki Miyahara               1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture                 Within Fujitsu Limited (72) Inventor Motohiko Fukuhara               1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture                 Within Fujitsu Limited (72) Inventor Katsunori Tanaka               1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture                 Within Fujitsu Limited                (56) Reference JP-A-4-165330 (JP, A)                 JP-A-5-203947 (JP, A)

Claims (7)

(57) [Claims] 1. A prism plate having a light emitting surface on an upper surface and a prism portion on a lower surface, a space formed on a lower surface side of the prism plate for confining light reflected by the prism plate, and a light in the space. The prism plate has a plurality of linear prisms with a triangular cross section.
At least one linear prism that is arranged in a number
In, the cross-sectional shape of the linear prism is
A surface emitting device, wherein the surface emitting device is formed so as to be partially different in the ridge direction of the . 2. The at least one linear prism
2. The surface emitting device according to claim 1 , wherein a cylindrical portion is formed in the ridgeline direction of the linear prism . 3. The at least one linear prism
There are, according to claim 1, characterized in that away portion-away middle is cut out of the ridge line direction of the linear prisms are formed
The surface emitting device described. 4. The surface emitting device according to claim 1, wherein the plurality of linear prisms are formed such that the prism angles are different in the ridge direction. 5.The upper surface is the light emitting surface and the lower surface is the prism part
And a prism plate, Formed on the lower surface of the prism plate and reflected by the prism plate
A space where the emitted light is confined, Having a light source for sending light to the space, the above The prism plate has the first linear prisms arranged in multiple rows.
Along the ridgeline of the first linear prism
Is cut out to form a notch, and the notch is
A second linear prism whose ridge direction is different from that of the first linear prism
The rhythm intersects at least one row with the first linear prism
It is formed bySideLight emitting device. 6. The light source is a cold cathode tube arranged laterally, and the prism plate has a configuration in which aligned linear prism portions are arranged in a direction orthogonal to the cold cathode tube. The surface emitting device according to any one of claims 1 to 5. 7. The surface emitting device according to claim 6, wherein the prism plate is configured such that a space below the prism plate becomes narrower as it goes away from the cold cathode tube. .